A DC/DC forward converter uses transformer windings to provide voltage conversion and galvanic isolation for the load. Power converters based on a forward topology are generally more efficient than flyback converters. By contrast with a flyback converter that stores energy as a magnetic field in the inductor when the switching element is conducting, a forward converter does not store energy in the transformer during the conduction time of the switching element. Instead, energy is passed to the output of the forward converter directly by transformer action during the switch conduction phase. The operation of the transformer in a forward topology doesn't inherently self-reset each power switching cycle. The transformer in a forward converter requires the application of a mechanism to reset the transformer each power cycle. The active clamp reset mechanism is presently finding extensive use.
FIG. 1 illustrates a conventional DC/DC forward converter 10 with an active clamp reset that converts an input voltage Vin into an output voltage Vout that may be higher or lower than the input voltage Vin. The converter 10 includes a power transformer PT having primary and secondary windings. The primary side and secondary side of the transformer PT may or may not be on opposite sides of an isolation barrier (i.e. referenced to independent grounds). A primary gate PG and an active clamp gate AG are arranged on the primary side of the transformer PT. A forward gate FG and a synchronous gate SG are provided on the secondary side of the transformer PT. The AG, FG and SG may be N-type MOSFETs, whereas the AG may be a P-type MOSFET.
The converter 10 further includes a clamp capacitor CC arranged on the primary side of the transformer PT, and an inductor L and an output capacitor COUT coupled on the secondary side of the transformer PT. The active clamp gate AG is controlled using a level shift circuit including a capacitor C1, a Schottky diode D and a resistor R.
FIG. 2 shows timing diagrams that illustrate operation of the forward converter 10. A switching period of the forward converter 10 is defined by the period of a pulse width modulation (PWM) signal that controls switching of the AG, PG, FG and SG in the forward converter 10. The PWM period is composed of the on-time tON, and the core reset time required to reset the magnetic flux in the transformer core. The reset is performed when the AG is on. When the PWM signal goes high, the AG is turned off as represented by the rising edge on the AG timing diagram. In a DC/DC Forward Converter with Active Clamp Reset, it is commonly known that setting a delay between the turn-off of the AG and the turn-on of the PG decreases power loss due to switching the PG with lower drain-source voltage. To minimize power loss, the PG is turned on only after the potential at the SWP node coupled to the drain of the PG falls from a voltage of Vin/(1−D) to the input voltage Vin, where D is a duty cycle of the converter 10. The Vin/(1−D) voltage corresponds to the voltage at the fully charged clamp capacitor CC. The time for the potential at the SWP node to fall is a function of the magnetizing current of the power transformer PT and the capacitance of the PG and AG, and generally is in the range from 200 ns to 1 μs.
On the secondary side, the FG is turned on also after a delay. The SG and the FG are configured as make-before-break, so the SG is on (a high level on the SG timing diagram) until the FG turns on. After the FG turns on (the rising edge on the FG timing diagram), the SG turns off (the falling edge on the SG timing diagram). If the FG and the SG switch immediately when the AG turns off, significant power will be lost in the body diode of the SG. If the FG and the SG switch after the PG turns on, then the SG and the PG will be cross-conducting and will lose power due to shoot-through. Therefore, the FG delay from the time when the AG turns off to the time when the FG turns on must be shorter than the PG delay from the time when the AG turns off to the time when the PG turns on, but not short enough to lose significant power in the body diode of the SG.
As indicated above, the PG delay can be up to 1 μs in some cases. While this delay is good for efficiency, power is not transferred from the primary side to the secondary side during this time. Therefore, the PG delay limits the maximum achievable duty cycle of the converter 10. Generally, in single switch forward converters, the duty cycle is already limited to between 65% and 80% to allow sufficient “off” time for the active clamp to reset the magnetic flux in the transformer core. The extra delay before turning on the PG further reduces this duty cycle. With a common switching frequency of 200 kHz, the maximum achievable duty cycle could be lower than 50% for a 1 μs delay. The need for higher duty cycles in active clamp reset forward converters is becoming more common as input ranges are extending from a ratio of 1:2 for a 36V-72V system to 1:4 for a 9V-36V system, or even higher.
If the system requires a wide input range, the design can usually be modified to be made to work. However, there is a huge compromise in component selection. For example, consider a 9V-36V system with a 200 kHz switching frequency, 1 μs delay, and 70% maximum duty cycle. In this case, the maximum achievable duty cycle is 50%. Therefore, the transformer turns ratio could be selected such that when Vin is 9V a 50% or lower duty cycle can be achieved. This will result in a non-optimal turns ratio for power loss in the transformer. Additionally, this higher turns ratio presents higher voltage stress on the MOSFETs, particularly when Vin is 36V.
A competent power supply designer would not choose the turns ratio based on the 1 μs delay setting, as it would cause more power loss than it saves. Therefore, the designer would simply reduce the delay setting to achieve the required duty cycle, and the power loss due to switching the PG would not be minimized.
Hence, there is a need for a technique that would allow the achievable duty cycle of a forward converter with an active clamp reset to be extended so as to minimize power loss.